In the process of establishing an oil or gas well, the well is typically provided with an arrangement for selectively excluding fluid communication with certain zones in the formation to avoid communication with undesirable fluids. A typical method of controlling the zones with which the well is in fluid communication is by running well casing down into the well and then sealing the annulus between the exterior of the casing and the walls of the wellbore with cement. Thereafter the well casing and cement may be perforated at preselected locations by a perforating gun or the like to establish fluid communication with product bearing zones in the formation. The cement also prevents the fluids in adjacent zones which are otherwise sealed from the zone of interest by a shale, a fault, or other geological condition from bypassing the geological seal by moving along the wellbore or well casing. Unfortunately, if the casing is not centered in the wellbore and is in contact with the walls of the wellbore, those portions of the casing string do not get surrounded by cement and thus do not seal the wellbore from migrating fluids. It may be desirable for other reasons to center a pipe string or otherwise space a pipe string from the walls of a borehole such as to facilitate perforating into the borehole.
A number of devices, which are typically called centralizers, have been developed to space the pipe string from the walls of the wellbore. An example of a typical centralizer is a bow spring centralizer which comprises a plurality of elongate spring metal strips which bow outwardly from the pipe string. The bow springs are typically provided at the collars of the well casing in sets to push the casing away from the walls of the wellbore. However, during installation of the string into the wellbore the bow springs create substantial frictional forces reducing the potential reach of a well. Also, the bow springs are somewhat fragile and subject to failure.
Another example of a centralizer for cementing operations is U.S. Pat. No. 2,654,435 issued on Oct. 6, 1953 to Oliver. The Oliver device comprises a shoe attached to the end of the casing string wherein the shoe includes bow springs which are held in a collapsed position by a stem extending through the wall of the shoe to an interior retainer. When the string is in the desired position in the wellbore, the casing string is pressurized to force a plug from an aperture in the end of the shoe. The plug is connected to the retainer which releases from the stem when the plug is forced from the aperture which releases the bow springs to centralize the casing. As an alternative arrangement two or more shoes could be installed in the same string with the retainers connected along a shaft to the end plug. Clearly, this system comprises a complicated deployment apparatus which may be subject to failure or premature deployment. Moreover, it would be impractical for a large number of centralizing shoes to be installed in a casing string which may be necessary in a horizontal well since it must rely on the one plug.
A series of patents has issued to Zandemer and others which involve sleeves extendable by hydraulic force to provide a conduit between the borehole wall and a casing or the like. These sleeves may be extended prior to cementing and then typically have portions that are dissolved by acid or the like after the cement is set to provide a flow path from earth formations into a pipe. These patents include U.S. Pat. Nos. 2,707,947; 2,855,049; 3,326,291; 3,347,313, 3,358,770, 2,775,304, 3,347,317 to Zandemer; and U.S. Pat. No. 3,924,677 to Prenner et al. The devices shown in these patents have various drawbacks that render them unacceptable for the situation presented in this application. For example, these devices do not provide a centralizer that will actually force the pipe to the center of the hole, but for the most part, the sleeves are extended until they engage the borehole and at that point they are locked in place. In most cases, nothing insures that each sleeve will extend to a fixed predetermined position. In addition, these devices for the most part do not have pistons with an outer end that resides within the maximum exterior profile of the pipe string. In addition, many of these devices also do not leave a full opening in the casing string when the sleeves are extended to allow devices to be run through the pipe string.
Accordingly, it is an object of the present invention to provide a new and improved method and apparatus for centralizing pipes in a wellbore which overcomes or avoids the above noted limitations and disadvantages of the prior art.
It is a further object of the present invention to provide a method and apparatus for fixedly spacing a pipe string from the walls of a wellbore which includes an extendable piston that remains within the maximum exterior profile of the pipe string while the pipe string is moved into and through the wellbore, including horizontal boreholes, and then is extendable to a predetermined fixed position by the application of a predetermined force to an inner end of the piston which extends into the pipe string while the pipe string is being run into the wellbore.
It is an additional object of the present invention to provide a method and apparatus for spacing a pipe from the walls of a wellbore by using extendable pistons that move when extended to a predetermined fixed position wherein the pistons are clear of the bore of the pipe string when the pistons are extended. The centralizing pistons may subsequently be used for fluid communication between the pipe and the formation, or for facilitating other borehole operations such as perforating, fracturing and treating. The extension force in movement of the pistons to an extended position needs to be accurately determined so that the apparatus is operable in response to a predetermined force.